Volumetric water content of soil or other materials may be measured using a probe having one or more capacitance-based moisture sensors. Each capacitance-based moisture sensor may include two conductive rings spaced by a dielectric to form a capacitor. The sensors may be arranged and fastened inside a probe which may be a plastic access tube that may be inserted in the soil. The electric field of each capacitor extends beyond the access tube and interacts with the soil around the tube to make the value of the capacitor variable. The variable capacitor may be coupled with a fixed inductor to form a free running oscillator with a frequency F=1/(2π√{square root over (LC)}).
An example of such a device is shown in U.S. Pat. No. 5,418,466 entitled “Moisture and Salinity Sensor and Method of Use” relating to moisture and salinity measurement and in particular to a sensor and its method of use which may provide values for the moisture/complex dielectric constant of a variety of mediums. A sensor apparatus is arranged for indicating the complex dielectric constant and conductivity of a medium and uses a tuned circuit. The tuned circuit oscillates such that the frequency of oscillation is representative of the complex dielectric constant of the medium.
Capacitance-based moisture sensors must be calibrated. Calibration needs to be done for each different sensor design and soil type. Calibration may be accomplished by measuring the frequency response of a representative sensor to a specified soil at several different volumetric water contents. The calibration process may involve installing one or more sensor assemblies in an open field of a specified soil type and monitoring the sensor response as water is added to the soil. The process may require stabilizing the soil for several days or weeks before each of the soil samples may be physically removed and analyzed to establish a fitted response curve to the soil readings. At each of the various moisture contents, the soil may be removed and sectioned with earth moving equipment to obtain a physical soil sample for volumetric analysis. This calibration process may be time and labor intensive. A more efficient and cost effective calibration apparatus and method is needed for capacitance-based moisture sensors for soil or other materials.
Migration mediums have been proposed for use with capacitance-based moisture sensors in an effort to reduce or eliminate calibration requirements. For example, U.S. Pat. No. 7,042,234 entitled “Soil Matric Potential and Salinity Measurement Apparatus and Method of Use” relates to a soil measurement arrangement that includes a capacitive based soil moisture and salinity sensor, a predetermined moisture migration medium located in a volume adjacent said sensor so that the medium substantially occupies the field of influence of said sensor. The medium is in moisture communication with the soil to be measured, and said sensor is adapted to measure and produce data representative of the volumetric water content of said medium. A sensor data processing means determines both the soil moisture and salinity of said medium. By using the measured volumetric soil moisture content and the moisture release curve of the medium, it is possible to derive the matric potential of the soil. The measured salinity of the medium corresponds to the salinity of the soil that is in moisture communication with the medium. However, a migration medium may increases the hole diameter for in-ground installation of the sensor and requires additional time and cost.
There is a need for a small capacitance-based moisture sensor and method for testing and characterizing soil types that is cost-efficient. There also is a need for an apparatus and method for calibration of capacitance-based moisture sensors that is small, cost-efficient, fast to operate and less labor intensive.